1. Field of the Invention
The invention relates to a four-stroke internal combustion engine with spark ignition, with direct injection of a fuel into a combustion space by means of an injection nozzle, with at least one inlet duct having an inlet valve and with at least one outlet duct having an outlet valve. The inlet valve and outlet valve are arranged on opposite sides of the combustion space, and the injection nozzle is arranged on the inlet-valve side. At least one recess is formed in a piston head on the inlet-valve side. A piston having a longitudinal center axis extending in the direction of movement of the piston is also provided. A recess is arranged on the inlet-valve side and designed, with respect to the inlet valve and injection nozzle, with at least one edge portion of the circumferential contour forming the highest point of the piston head, in such a way that an airflow entering the combustion space from the correspondingly designed inlet duct via the inlet valve and an injection jet entering the combustion space from the injection nozzle enter the recess on opposite sides. Furthermore, an elevation dividing the recess into a fuel recess and an air recess transversely to the injection direction is formed on the piston head, in such a way that the injection jet enters the fuel recess on a side located opposite the elevation and the airflow enters the air recess, in the opposite direction to the injection jet, on a side located opposite the elevation.
2. Discussion of the Prior Art
In order to generate a suitable mixture of air and fuel in a combustion space of a four-stroke internal combustion engine with direct injection, it is known to provide special inlet ducts generating tumble or swirl flows and to design piston heads with corresponding recesses and guide ribs. In a swirl flow, a cylinder charge rotates about the cylinder axis, for example by virtue of the shape of the inlet duct, while, in a tumble flow, rotation about an axis parallel to a crankshaft axis can be observed.
Thus, DE 197 13 030 A1, DE 197 13 029 A1 and DE 197 13 028 A1 each describe a four-stroke internal combustion engine with spark ignition and direct injection, a piston surface of each piston of a cylinder in each case having an H-shaped, T-shaped or U-shaped guide-rib arrangement.
EP 0 639 703 A1 and EP 0 694 682 A1 disclose in each case a further embodiment of an internal combustion engine with direct injection, in which a swirl flow is generated in the cylinder space due to the shaping of the inlet ducts. Here, the piston surface has a pronounced recess with a surrounding squeeze or squish area, the recess being arranged eccentrically in such a way that the spark plug located centrally in the combustion space and the radially arranged injection valve are in each case located at the recess edge. The fuel is sprayed directionally against the recess edge which is shaped for this purpose. The piston surface therefore has the task, here, of primarily atomizing the fuel jet. The swirl flow assumes the task of transporting to the spark plug the fuel which rebounds in atomized form from the recess.
EP 0 558 072 A1 discloses a further embodiment of an engine, in which a reversed tumbling movement of the combustion-space flow is generated as a result of the shape and arrangement of the inlet ducts and is reinforced by the piston surface being shaped in the manner of a ski jump. This piston surface serves at the same time for deflecting the injection jet to the spark plug which is arranged in the center of the cylinder. The injection jet and combustion-space flow thus sweep over the piston surface in the same direction. However, after impinging onto the cylinder head wall near the spark plug, the injection jet or the mixture cloud originating from it after deflection at the piston may spread out, virtually unimpeded, in all directions. An attempt at as high a concentration as possible of the mixture cloud after deflection at the piston therefore cannot be detected. Moreover, the ski jump formed on the piston surface generates a squish area below the outlet valve. This, admittedly, generates a desired additional flow movement shortly before top dead center during the compression of the engine. However, after the dead center has been passed, this flow movement is reversed, thus causing the mixture concentration built up during compression to be broken up.
Moreover, all these arrangements have the disadvantage that optimum mixture formation does not take place over an entire characteristic map range of a four-stroke internal combustion engine.
The object on which the present invention is based is to provide an internal combustion engine of the above-mentioned type with an improved piston, the above-mentioned disadvantages being overcome.
This object is achieved, according to the invention, by means of a four-stroke internal combustion engine of the above-mentioned type having a piston with a novel configuration.
For this purpose, there is provision, according to the invention, for, in the sectional plane, the fuel recess to have a depth tKa between the elevation and a lower fuel-recess edge of 4 mm to 12 mm. In the sectional plane, the air recess has a depth tL with respect to an air-recess edge located opposite the elevation of 2 mm to 10 mm. In the sectional plane, a tangent of a fuel-recess wall at a fuel-recess edge located opposite the elevation forms with the longitudinal center axis an angle xcex3K1 of 40 degrees to 62 degrees. In the sectional plane, a tangent of an air-recess wall at a fuel-recess edge located opposite the elevation forms with the longitudinal center axis an angle xcex3L1 of 30 degrees to 62 degrees.
The combination of these special geometric features results surprisingly in an optimum control of the stratified profile by means of an accurate coordination of the spatial arrangement of the injector or injection nozzle and the ignition point or spark plug within wide characteristic map ranges. The sensitivity of the mixture formation as regards production variations and use-related changes in the injector is reduced. The combustion profile exhibits continuous energy conversion, at the same time taking into account all the requirements of outstanding full-load potential. Furthermore, the invention can be integrated in a simple way into already existing multi-valve cylinder-head concepts.
In respective of preferred embodiments, the elevation projects above the piston head, the depth tKa is 11.1 mm, 11.41 mm, 11.47 mm or 4.6 mm, the depth tL is 8 mm, 3.07 mm, 2.5 mm or 8.5 mm, the angle xcex3L1 is 50.6 degrees, 54.2 degrees, 60.4 degrees or 34.8 degrees, the angle xcex3K1 is 56.4 degrees, 48.9 degrees, 49.8 degrees or 60.4 degrees. In still further embodiments, in a sectional plane, the air-recess edge located opposite the elevation is at a perpendicular distance xL from the longitudinal center axis of 33 mm to 36 mm, in particular 35.76 mm, 34.6 mm, 35.75 mm or 33.27 mm, in the sectional plane, the elevation is at a perpendicular distance xE from the longitudinal center axis of 0 mm to 7 mm, in particular 0.5 mm, 6.85 mm, 2 mm, 4.14 mm or 2 mm, in the sectional plane, a tangent of a fuel-recess wall at the elevation forms with the longitudinal center axis an angle xcex3K2 of 10 degrees to 50 degrees, in particular 20.9 degrees, 12.5 degrees, 18.7 degrees or 46.5 degrees, and/or, in the sectional plane, a tangent of an air-recess wall at the elevation forms with the longitudinal center axis an angle xcex3L2 of 0 degrees to 80 degrees, in particular 23.2 degrees, 64.6 degrees, 77.3 degrees or 47.7 degrees.
Further features, advantages and advantageous refinements of the invention may be gathered from the dependent claims and from the following description of the invention with reference to the accompanying drawings.